1. Field of the Invention
The present invention relates generally to an observation method and an observation apparatus for radio waves and acoustic waves, and more specifically, to an observation method and an observation apparatus which enables an observation without dead angle.
Also, the present invention relates to a multi-dimensional hologram data processing apparatus and a method for extracting a plurality of peak points for multi-dimensional hologram data and an area occupied thereby using the multidimensional hologram data processing apparatus.
2. Description of the Related Art
Since radio waves and acoustic waves are similarly waves, holograms of those waves can be observed as is the case with light and are utilized to visualize a wave source image and to specify noise (such as undesired electromagnetic wave radiation and ambient noise) sources. The inventors have disclosed a method and an apparatus for observing a radio wave hologram and acoustic wave hologram to derive a wave field intensity and wave source image, for example, in Japanese Laid-open Patent Application No. 8-201459 and Japanese Laid-open Patent Application No. 9-134113.
FIG. 1 is a schematic diagram for illustrating a hologram observation method described in Japanese Laid-open Patent Application No. 8-201459. Rectangular hologram observation surface 92 is set away from observation object (wave source) 91. Scanning sensor 93 which moves two-dimensionally in hologram observation surface 92 is used to detect radio waves and acoustic waves at a predetermined observation frequency from observation object 91 at each point in hologram observation surface 92. In addition, fixed sensor 94 is provided separately from scanning sensor 93 and is used to similarly detect radio waves and acoustic waves at the above-mentioned predetermined frequency from observation object 91. Signals from both sensors 93 and 94 are interfered with at interference unit 95 and the signals after interference is detected by detector 96. The detected signal (signal representing the correlation of the signals from both sensors 93 and 94, i.e. signal representing the hologram intensity at a position of scanning sensor 93 in hologram observation surface 92) is stored in memory 97 corresponding to the coordinates of scanning sensor 93 in hologram observation surface 92. When observations are completed at all observation points in hologram observation surface 92, data is read from memory 97 to reconstruct a hologram image by image reconstructing unit 98.
In the prior art hologram observation method as described above, however, plane scanning in the hologram observation surface is used to observe holograms, so that it is not possible to observe radio waves and acoustic waves arriving from the backside of the hologram observation surface. Additionally, it is difficult to observe radio waves and acoustic waves of extremely oblique incident angle with respect to the hologram observation surface. Thus, in reality, a field of view angle is as small as 120 degrees and the remaining angle distance of 240 degrees is a dead angle, which causes a disadvantage of a limited observation. A required observation can be made even with a relatively small field of view angle as described above, for example, when an observation object is placed at a corner of a room such as radio wave darkrooms and a hologram observation apparatus is placed at a corner opposite to the observation object. However, when an observation is made outdoors, radio waves and acoustic waves to be observed can not arrive only in a front direction. For this reason, many components of radio waves and acoustic waves, arriving other than in the front direction, are left without being observed, thus producing some space which can not be observed.
Furthermore, the prior art hologram observation method has a disadvantage of lacking a real-time basis observation since the hologram image is reconstructd after the data at all the observation points is acquired in the hologram observation surface.
Also, there are a circumference scanning type hologram observation for extracting each arrival angle (.theta., .phi.') of a plurality of waves and a plane scanning type hologram observation for extracting each of the coordinates (X.sub.s,Y.sub.s,Z.sub.s) of a plurality of points wave sources illustrated in FIG. 2 (Hitoshi Kitayoshi: "Study for visualizing electromagnetic radiation and propagation", second chapter "principle of visualization and reconstruction algorithm" in doctoral dissertation in Tohoku University, February 1997).
Hologram observation data has an accuracy equal to or greater than the observed dimensions, for example, a three-dimensional image can be reconstructd from data recorded in a two-dimensional plane. However, the reconstructd image has a limited resolution due to a limitation of an observation surface as described in the above-mentioned literature for the plane scanning type hologram observation (Hitoshi Kitayoshi: "Study for visualizing electromagnetic radiation and propagation", second chapter "principle of visualization and reconstruction algorithm" in doctoral dissertation in Tohoku University, February 1997, pp.13-19). Thus, when a plurality of wave sources are simultaneously observed, it is substantially difficult to automatically extract the position and intensity of each wave source.
Conventionally, a contour line processing method and a path survey method are used to detect peaks and an area occupied by the peaks as shown in FIG. 3. The path survey method is one for surveying the negative inclination path from a peak point in all moving directions to determine an area occupied by the point.
The above-mentioned prior art has disadvantages as described below.
Specifically, in the above-mentioned algorithm, the creation of a path for survey is complicated and is not easily implemented by simple hardware or digital signal processing (DSP).
Although another approach is also contemplated in which a reconstructd image is improved in extended peak (blurred image) by modifying a reconstructing algorithm for hologram images, the approach is not complete in the relationship between parameters used when applying the algorithm and the stability of the reconstructd image (Hitoshi Kitayoshi: "Study for visualizing electromagnetic radiation and propagation", second chapter "principle of visualization and reconstruction algorithm" in doctoral dissertation in Tohoku University, February 1997, pp.20-35). The parameter refers to a threshold value in SPIM (Spectrum Phase Interpolation Method) (Hitoshi Kitayoshi: "higher resolution for short time frequency spectrum analysis" Shingakuron A, vol.J76-A, no.1, pp.78-81, January 1993., Hitoshi Kitayoshi: "higher resolution for two-dimensional complex spectrum analysis" Shingakuron A, vol.J76-A, no.4, pp.687-689, April 1993), while the parameter refers to a filter terms number and the like in MEM (Maximum Entropy Method) (Mikio Hino: "Spectrum analysis" Asakura Syoten, 1977, Yoshinao Aoki: "Wave signal processing" Morikita Pub., sixth chapter "Maximum Entropy Method", 1986).